1. Field of the Invention
The present invention relates to a semiconductor device using TFTs (thin film transistor) mounted on an insulating substrate such as a glass and more particularly to a semiconductor device utilizable for an active matrix type liquid crystal display.
2. Description of the Related Art
A semiconductor device having TFTs on an insulating substrate such as a glass is known to be utilized in an active matrix type liquid crystal display, image sensor and the like using such TFTs for driving picture elements.
Generally a thin film silicon semiconductor is used for the TFT used in such devices. The thin film silicon semiconductor may be roughly classified into two semiconductors; those composed of amorphous silicon (a-Si) semiconductor and those composed of silicon semiconductor having a crystallinity. The amorphous silicon semiconductor is most generally used because its fabrication temperature is low, it can be fabricated relatively easily by a vapor phase method and it has a mass-producibility. However, because it is inferior as compare to the silicon semiconductor having a crystallinity in terms of physical properties such as an electrical conductivity, it has been strongly demanded to establish a method for fabricating a TFT composed of the silicon semiconductor having a crystallinity to obtain a faster characteristic. By the way, as the silicon semiconductor having a crystallinity, there are known to exist a polycrystal silicon, microcrystal silicon, amorphous silicon containing crystal components, semi-amorphous silicon having an intermediate state between crystallinity and amorphousness.
The following method is known to obtain those thin film silicon semiconductors having a crystallinity: (1) directly form a film having a crystallinity, (2) form an amorphous semiconductor film and crystallize it by energy of laser light, and (3) form an amorphous semiconductor film and crystallize it by applying thermal energy.
However, it is technically difficult to form a film having favorable physical properties of semiconductor on the whole surface of a substrate by the method of (1). Further, it has a problem in terms of cost that because its film forming temperature is so high as more than 600° C., a low cost glass substrate cannot be used. The method (2) has a problem that its throughput is low because an irradiation area is small when an eximer laser which is presently most generally used is used. Further, the laser is not stable enough to homogeneously treat the whole surface of a large. area substrate. Accordingly, it is thought to be a next generation technology. Although the method (3) has a merit that it allows to accommodate with a large area as compare to the methods (1) and (2), it is also necessary to apply such a high temperature as more than 600° C. as the heating temperature. Accordingly, the heating temperature needs to be reduced in a case of using a low cost glass substrate. In particular, because the screen of present liquid crystal display is enlarged more and more, a large size glass substrate needs to be used accordingly. When such a large size glass substrate is used, its contraction and strain caused during the heating process indispensable in fabricating the semiconductor produce a large problem that they reduce an accuracy of mask positioning and the like. In particular, because the strain point of the 7059 glass which is presently most generally used is 593° C., it deforms largely by the conventional heating crystallization method. Further, beside the problems concerning to the temperature, it takes more than tens of hours as the heating time required for the crystallization in the conventional process, so that such time needs to be shortened.